Hydrophilic polymer based on acrylic acid and alkali metal acrylate, its preparation process and its use as an absorbing agent, in particular as an absorbing agent for articles of hygiene

ABSTRACT

Hydrophilic polymer, as microbeads insoluble in water, based on acrylic acid and an alkali metal acrylate constituted by an acrylic acid-potassium acrylate copolymer, cross-linked with 100 ppm to 3,000 ppm relative to the total weight of monomers of one or more cross-linking agents chosen from the group constituted by the products of general formula I: ##STR1## A=a bivalent radical derived from a C 3  -C 6  alkane presenting an absorption capacity for a salty physiological solution of 30 to 70 g per gram and from 10 to 35 g per gram under pressure, preparation process and use notably in articles of hygiene.

This is a division of application Ser. No. 07/494,366 filed Mar. 16,1990.

The present invention relates to a hydrophilic polymer with a highabsorption capacity for saline aqueous solutions, based on acrylic acidand alkali metal acrylate, its preparation process and its applicationas an absorbing agent, notably as an absorbing agent for babies nappies.

Polymer substances capable of absorbing several times their own weightof water and converting it into a gel are known. These hydrophilicpolymers are either natural or semi-synthetic polymers such asderivatives of cellulose, starch, alginate, polysaccharides, orsynthetic polymers based notably on maleic acid or (meth)acrylic acid.

Thus a large variety of hydrophilic polymers are known, but foragricultural, horticultural or sanitary use, products are always beingsought which offer an increased absorption capacity not only for water,but also for water charged with electrolytes, with a rapid speed ofabsorption and good powers of retention in the gel state.

Moreover, for hygienic use, notably in the manufacture of objectsintended to be in contact, in a damp state, with human skin, productsare sought that are atoxic, of low cost, do not present syneresis,contain neither nitrated derivatives nor residual monomers andsimultaneously possess an increased absorption capacity both unloadedand loaded with saline aqueous solutions, In effect, for example, one ofthe important problems to resolve in the manufacture of babies nappies,is the retention of urine by the nappy, for several hours, attemperatures that may reach 40° C. and despite the pressure caused bythe baby's weight. A partial solution to these problems has beenprovided by the teaching of European Patent No. 0,083,022 whichrecommends a post-cross-linking of the resin absorbing the water with across-linking agent having at least two functional groups which canreact with the carboxylate groups or the groups present in thiscarboxylate-group resin, such as hydroxyl, amino or sulpho groups in thepresence of 0.01 to 1.3 parts by weight of water per part by weight ofresin. This solution, though intersting, is expensive to employ: itrequires, in effect, the preparation, in a first stage, of thewater-absorbing resin then, in a second stage, the subjection of thisresin to a post-cross-linking in particular conditions.

In conclusion, it can be said that today no known hydrophilic polymersabsorbing salty water respond to the needs expressed by the market.

Now the Applicant has surprisingly discovered such a product. Thepresent invention therefore relates to a hydrophilic polymer, in theform of microbeads insoluble in water, based on acrylic acid and alkalimetal acrylate, characterised in that it is composed of a potassiumacrylate-acrylic acid copolymer, containing, in molar proportions, from55 to 85% of potassium acrylate, cross-linked with from 100 ppm to 3,000ppm relative to the total weight of monomers of one or morecross-linking agents chosen from the group constituted by the productsof general formula I: ##STR2## in which A represents a bivalent radicalderivative of a C₃ -C₆ alkane branched or not, by the removal of ahydrogen atom from each of the two terminal carbon atoms such as thefollowing radicals; trimethylene, tetramethylene, ethylethylene,proylene, 2,2-dimethyl-1,-3 propanediyl and that it presents anabsorption capacity for a salty physiological solution of the order of30 to 70 g per gram and of the order of 10 to 35 g per gram under apressure of 15 g per cm².

By "microbeads", one means approximately spherical beads of a diameterof between 0.05 and 1 mm.

By "insoluble in water", one means that the polymers contain, at ambienttemperature, less than 5% of products soluble in water.

These copolymers are preferably cross-linked with 200 to 3,000 ppm andnotably 200 to 1,000 ppm relative to the weight of the monomers ofbis(oxiranylmethoxy)-1,4-butane, currently designated 1,4-butanedioldiglycidyether.

Among the copolymers defined above, more particularly the acrylicacid-potassium acrylate copolymers containing in molar proportions from65 to 80% of potassium acrylate may be cited.

By "salty physiological solution" one designates an isotonic solutioncontaining 9 g of sodium chloride per liter of distilled water.

According to the invention, the copolymers defined above are able to beprepared by a polymerisation process in a water-in-oil suspensioncarried out in an inert atmosphere, characterised in that an aqueousphase containing the cross-linking agent or agents of general formula(I), one or more hydrosoluble polymerisation initiators which generatefree radicals, the chosen monomers and water in a quantity such that themonomer concentration is 55±10% by weight are introduced slowly, withagitation, into the deoxygenated oil phase, maintained at boiling pointand containing a colloid protector, then when the polymerisationreaction is finished, 75 to 90% of the water introduced is eliminated byazeotropic distillation and then the polymer formed is isolated, eitherby filtration followed by drying at 110°±10° C. for at least 30 minutes,or by direct azeotropic drying at 110°±10° C. for at least 30 minutes insuch a manner so as to obtain micropearls of the sought hydrophilicpolymer containing 7±3% of water by weight.

According to a variation of the process described above, thecross-linking agent or agents of general formula (I) used can be placedin total or in part in the oil phase.

By "deoxygenated oil", one means a virtually perfectly or preferablyperfectly deoxygenated oil.

The oil phase is principally constituted by one or more hydrocarbons,non-miscible with water, inert with respect to the polymerisationinitiators and able to produce an azeotrope with water, such ascyclohexane or petrol fractions with boiling points of between 50° and180° C.

The ratio by weight of the oil phase to the aqueous phase isadvantageously between 0.9 and 1.1.

The polymerisation reaction is conducted under reflux of the reactionmedium, usually at ambient pressure. It can equally be achieved at alower or higher pressure to that of ambient pressure.

Preferably the colloid protector used at a dosage of 0.5 to 2% by weightrelative to the weight of the monomers is chosen from among thosecurrently used in this type of polymerisation in suspension (cfKirk-Othmer, Encyclopaedia of Chemical Technology, 3rd edition, volume1, page 400). Advantageously, one will chose a cellulose ether and,preferably, a cellulose ethylether presenting an ethoxyl proportion of48 to 49.5% (cf Encyclopaedia of Polymer Science and Engineering, 2ndedition, volume 3, page 254). The colloid protector is dissolved ordispersed beforehand in the oil phase.

The polymerisation reaction is started with one or more hydrosolublefree radical generators, offering a half-life at 70° C. greater than twohours. Advantageously, the polymerisation reaction is initiated with amixture of at least two different hydrosoluble initiators such asdefined previously of which at least one is chosen from among themineral peroxides, and at least one is chosen from among theazo-compound derivatives. They are advantageously used at theconcentration of 200 ppm to 3,000 ppm relative to the weight of themonomers and advantageously from 500 to 1,000 ppm. Such initiatingagents are notably certain mineral peroxides such as sodiumperoxodisulphate or certain azo-compounds such as 4,4'-dicyano-4,4'-azo-dipentanoic acid. The initiating agent or agents used are dissolved inthe water then this solution is carefully deoxygenated.

The potassium acrylate is advantageously obtained in aqueous solution,by direct salification of an aqueous solution of acrylic acid withpotash. This salification is advantageously effected at a temperature ofbetween 20° and 35° C. The monomers used are dissolved in the water,preferably at a concentration of approximately 55±10% by weight.

The aqueous solution of initiator or initiators and the aqueous mediumcontaining the monomers in solution, and optionally the chosencross-linking agent or agents, are advantageously introduced into theoil phase after extemporaneously mixing during their introduction, insuch a manner that they only remain in contact for a few seconds.

The duration of the introduction can vary according to the operatingunits, but generally it is comprised between one and two hours. At theend of the introduction it is advantageous to maintain the reactionmedium at boiling point under agitation so as to perfect thepolymerisation.

During the polymerisation reaction, the copolymers formed spontaneouslycross-link with each other to produce cross-linked copolymers insolublein water, of high hydrophilic power and with a very low proportion ofresidual monomers, which are always lower than 0.01% by weight relativeto the weight of polymer.

The spontaneous cross-linking is accordingly more favoured the lower thedegree of neutralisation of the acrylic acid and the higher thepolymerisation temperature. The copolymers of the present invention arethus cross-linked, on the one hand by thermal means and on the other, bychemical means by the cross-linking agent or agents of general formula(I).

In order to develop all its properties, the hydrophilic polymeraccording to the present invention should be dried for at least 30minutes at a temperature of 110°±10° C., in such a way so as to obtain ahomogeneous drying even in the interior of the beads.

It is very surprising that a hydrophilic polymer presents with respectto salty water a high absorption capacity, just as well as under theinfluence of a pressure of 15 g/cm², for a person skilled in the art,these two properties are antinomic. It is in effect known that to obtaina hydrophilic polymer presenting a high absorption capacity vis-a-vissalty water under pressure, this polymer must be highly cross-linked, inorder not to be crushed and, reciprocally, a high absorption capacity ina state such as is obtained with a very weakly cross-linked polymer. Nowthe hydrophilic polymers of the present invention simultaneously possesthese two properties, which indicates that the microbeads of the polymerare weakly cross-linked at the centre and highly cross-linked at thesurface. One could therefore say that the micropearls of the polymersaccording to the present invention present a "core-shell" structure.

The water absorption capacity of the polymer is determined at 20° C., byagitating for 30 minutes 0.4 g of polymer in 500 g of water, then byweighing the drained polymer gel. The weight found is related to 1 g ofpolymer. The copolymers of the present invention show in this test anabsorption capacity of the order of 250 to 750 g per gram.

The salty physiological solution absorption capacity of the polymer isdetermined at 20° C., by agitating for 30 minutes 2 g of polymer in 500g of salty physiological solution, then by weighing the drained polymergel obtained. The weight found is related to 1 g of polymer, aspreviously. The copolymers of the present invention show in this test anabsorption capacity of the order of 30 to 70 g per gram.

The speed of absorption of a salty physiological solution by the polymeris determined at 20° C., in a 100 ml beaker, with a diameter of 55 mmand a height of 70 mm, by agitating at a speed of 600 revolutions perminute with a magnetic agitator equipped with a 25 mm long magnetisedbar, 2 g of polymer in 50 g of salty physiological solution and bymeasuring the time required for the vortex to disappear. In this test,the copolymers of the present invention require 50 to 80 seconds for thevortex to disappear.

The proportion of extractables is determined according to the followingmethod:

1 g of the polymer to be tested is placed in 200 g of saltyphysiological solution;

this suspension is agitated for 16 hours at 20° C., then it is filtered;

the carboxylic and carboxylate functions present are analysed on 100 cm³of the filtrate;

the result of this analysis is expressed in grams of polymer dissolvedfor 100 g of dry polymer.

In this test, the copolymers of the present invention show a proportionof extractables of 1 to 5%. At the end of the polymerisation reaction,the reaction solvents are eliminated by azeotropic distillation until asuspension showing a proportion of dry content of 80±10% is obtained,then the suspension is filtered and the precipitate collected and driedto 90-95% dry content.

The absorption capacity by capillary action under a pressure of 15 g/cm²is determined at 20° C. according to the following protocol: 40 g ofFontainebleau sand with a granulometry of 0.100 to 0.300 mm, 2 g ofcopolymer to be tested and finally 40 g of Fontainebleau sand are spreadsuccessively and uniformly in a cylindrical funnel with a filteringplate of 90 mm in diameter and a porosity of 1. Then on the top layer ofsand, using a glass disc of 90 mm diameter as an intermediary, a totalload of 954 g is placed, then the funnel is plunged into a vatcontaining a salty physiological solution, at a constant level, in amanner so that the water level rigorously remains level with the upperface of the sintered glass and the quantity of salty physiologicalsolution absorbed by the copolymer by capillary action in 90 minutes ismeasured.

The result is expressed in g of salty physiological solution per gram ofdry polymer.

The dry extract is determined by drying a sample of constant weight at140° C. and it is expressed in percentage by weight of dry productcontained in the crude product.

The copolymers of the present invention therefore present interestingabsorbent properties which justify their application as absorbingagents. Thus equally a subject of the invention is the polymers asdefined previously, as absorbing constituents, preferably in an articleof hygiene, notably for the manufacture of babies nappies, articles forincontinents or sanitary towels.

The following examples illustrate the invention without however limitingit:

EXAMPLE 1

In an inert atmosphere, 3.5 g of cellulose ethylether, containing from48 to 49.5% of ethoxylated groups and showing at 25° C. a viscosity of200 mPa.s determined in solution at 5% in a toluene-ethanol mixture80-20 by weight, is dispersed in 618.5 g (7.35 moles) of cyclohexane.

In to this dispersion, carefully deoxygenated, agitated and maintainedat boiling point, is introduced over 90 minutes, in an inert atmosphere,a solution obtained by extemporaneously mixing during the introductionof the following:

on the one hand a deoxygenated solution of:

0.106 g of sodium peroxodisulphate,

0.212 g of dicyano-4,4'-azo-4,4' dipentanoic acid dissolved in 10 g of0.2N potash;

on the other hand, a solution prepared extemporaneously

by dissolving at a temperature lower than 30° C.: 230 g (3.192 moles) ofacrylic acid in 417.4 g of an aqueous solution of potash containing128.9 g (2.297 moles) of potassium hydroxide,

then by adding into this solution 319 mg of bis(oxiranylmethoxy)-1,4butane.

The suspension obtained is then maintained for one hour at boiling pointunder agitation, then it is subjected to an azeotropic distillation witha recycling of the cyclohexane until the elimination of approximately85% of the water present, and finally it is cooled down to ambienttemperature and filtered. The precipitate collected is then dried at110° C. for 60 minutes in a ventilated heating chamber. Thus ahydrophilic polymer is obtained in the form of white microbeads,insoluble in water, showing the following characteristics:

dry extract: 92%

water absorption capacity: 330 g per gram;

salty physiological solution absorption capacity:

as is: 44 g/gram,

under a pressure of 15 g/cm² : 23 g/gram;

proportion of extractables: 2% by weight;

proportion of residual monomers: less than 0.005% by weight;

speed of absorption of a salty physiological solution: 70 s;

granulation: 0.1-0.8 mm.

EXAMPLE 2

Operating as in example 1 but using 208 mg instead of 319 mg ofbis(oxiranylmethoxy)-1,4 butane.

A hydrophilic polymer is thus obtained, as microbeads, showing thefollowing properties:

dry extract: 92%

water absorption capacity: 380 g/gram;

salty physiological solution absorption capacity:

as is: 47 g/gram,

under a pressure of 15 g/cm² : 26 g/gram;

proportion of residual monomers: less than 0.005% by weight.

EXAMPLE 3

Operating as in example 1 but using 159 mg instead of 319 mg ofbis(oxiranylmethoxy)-1,4 butane.

A hydrophilic polymer is thus obtained, as microbeads, showing thefollowing properties:

dry extract: 92%

water absorption capacity: 485 g/gram;

salty physiological solution absorption capacity:

as is: 54.5 g/gram,

under a pressure of 15 g/cm² : 27.5 g/gram;

proportion of extractables: 1.6%;

proportion of residual monomers: less than 0.005% by weight.

EXAMPLE 4

Operating as in example 1 but using 300 mg instead of 319 mg ofbis(oxiranylmethoxy)-1,4 butane which is placed in the oil phase.

A hydrophilic polymer is thus obtained, as microbeads, showing thefollowing properties:

dry extract: 92%;

water absorption capacity: 340 g/gram;

salty physiological solution absorption capacity:

as is: 46.5 g/gram,

under a pressure of 15 g/cm² : 24 g/gram;

proportion of residual monomers: less than 0.005% by weight.

EXAMPLE 5

Operating as in example 1 but using 256 mg instead of 319 mg ofbis(oxiranylmethoxy)-1,4 butane of which 128 mg are placed in the oilphase and 128 mg are placed in the aqueous phase containing themonomers.

A hydrophilic polymer is thus obtained, as microbeads, showing thefollowing properties:

dry extract: 92%;

water absorption capacity: 320 g/gram;

salty physiological solution absorption capacity:

as is: 46 g/gram,

under a pressure of 15 g/cm² : 27 g/gram;

proportion of residual monomers: less than 0.005% by weight.

It goes without saying that the present invention has been describedonly in a purely explanatory and in no way limiting manner and that anymodification, in particular with regard to technical equivalence, can bemade without being beyond its scope.

We claim:
 1. A process for the preparation of a water-insoluble hydrophilic polymer, in microbead form, comprising an acrylic acid/potassium acrylate copolymer containing 55-85 molar percent potassium acrylate, cross-linked with 100-3,000 ppm of a cross-linking agent of the formula ##STR3## in which A is a bivalent radical derived from a C₃ -C₆ alkane, comprisingeffecting polymerization in a water-in-oil suspension under an inert atmosphere, wherein an aqueous phase containing a cross-linking agent of said formula, at least one hydro-soluble polymerization initiator which generates free radicals, the monomers and water in a quantity such that the concentration of monomers is from 55%±10% by weight are slowly introduced, with agitation, into a deoxygenated oil phase maintained at boiling and containing a colloid protector, eliminating 75-90% of the water introduced by azeotropic distillation on completion of the polymerization reaction, isolating the formed polymer by filtration followed by drying at 110°±10° C. for at least thirty minutes or by direct azeotropic drying at 110°±10° C. for at least thirty minutes so as to obtain said microbeads containing 7%±3% by weight of water.
 2. Process according to claim 1, characterised in that the cross-linking agent or agents of general formula (I) are placed in total or part in the oil phase.
 3. Process according to claim 1, characterised in that the polymerisation is initiated by a mixture of at least two initiating agents, hydrosoluble and different, presenting a half-life duration at 70° C., of greater than 2 hours, one of which at least is chosen from mineral peroxides and one at least is chosen from the hydrosoluble azo-compound initiating agents.
 4. Process according to claim 2, wherein said polymerization is initiated by a mixture of at least two initiating agents, hydrosoluble and different, presenting a half-life duration at 70° C., of greater than two hours, one of which at least is selected from the group consisting of mineral peroxides and one at least of which is selected from the group consisting of hydrosoluble azo-compound initiating agents. 